Technical Field
The present disclosure relates to a magnet-free rotating electric machine, and more particularly to a double-stator type magnet-free rotating electric machine including an outer stator on the outer circumferential side of an annular rotor and an inner stator on the inner circumferential side of the annular rotor.
Related Art
As a magnet-free rotating electric machine having a rotor with no permanent magnets, a rotor structure is well-known in which a rotor is provided with rotor salient poles around which rotor coils are wound such that induced current is generated by the field from a stator and the rotor salient poles are magnetized by this induced current.
JP 2010-279165 A describes that, for a magnet-free rotating electric machine, in addition to prior art methods in which coils are wound around rotor salient poles, another method is available in which coils are wound around a rotor yoke. JP 2010-279165 A also describes a structure in which rotor salient poles include induction coils positioned near a gap with a stator, and separately from these induction coils, common coils are provided away from the stator. JP 2010-279165 A further describes another structure in which induction coils are wound around rotor salient poles and a common coil is wound around a rotor core.
As techniques relating to the present disclosure, JP 2011-244643 A discloses a double-stator type motor in which permanent magnets magnetized in the circumferential direction are positioned around an annular rotor. An inner stator is provided inside the rotor and an outer stator is provided outside the rotor. The inner stator and the outer stator respectively include Y-connected three-phase coils. In this disclosure, the number of magnetic poles of the inner stator and the number of magnetic poles of the outer stator are equal to each other. The permanent magnets disposed on the rotor apply magnetic flux in parallel to both of the inner stator and the outer stator. Magnet-free rotating electric machines are advantageous in costs because expensive magnetic materials are not required. However, because of a risk that the coils wound around the rotor salient poles may be detached from the rotor salient poles due to a centrifugal force of the rotating rotor, structure becomes complicated by use of coil holders or the like for avoiding such detachment. Such a complicated structure limits the space for coil windings, resulting in lowered output efficiency of the rotating electric machine.